1. Field of the Invention
The present invention relates to a method and system for the optical remote measurement of the air pressure by laser excitation, particularly in front of an aircraft, in a zone undisturbed by its movement.
The air pressure is one of the parameters which are used, for controlling and piloting the aircraft, to elaborate then display certain parameters of the aircraft, such for example as its air speed, the mach number, its ground speed, its altitude, its power setting.
A conventional anemometer, called dynamic airspeed indicator, for example comprises static pressure intakes and a Pitot tube for total pressure intake. But the measurement of a dynamic airspeed indicator suffers from errors due to aerodynamic disturbances caused by the movement of the aircraft. It was then a priori interesting to invent an optical pressure measurement eliminating the drawbacks of conventional barometry, in order to obtain the true pressure of the ambient air.
2. Description of the Prior Art
From the document US-A-4 493 553 a method is already known for the optical remote measurement of the air pressure by laser excitation in which, having determined the air temperature, by means of a laser generator with very fine spectral line and by excitation of an absorption spectral line of molecular oxygen, backscattered radiation is caused, the backscattered energies are measured at two different distances, the ratio of these energies is determined, a coefficient of absorption is derived therefrom depending on the temperature and the molecular density of oxygen and the density and so the pressure is calculated.
But the spectral distribution of the outward and inward radiation is not the same, so that the coefficient of attenuation emitted radiation and the coefficient of attenuation of the backscattered radiation may not be the same. The pressure measurement would then be erroneous.
Also forming part of the state of the art is the article :"Proposed single pulse two dimensional temperature and density measurements of oxygen and air, Miler et al., March 1988, vol 13, no. 3, Optics Letters". This document teaches the use of an ArF laser tuned to a UV transition of oxygen while simultaneously observing the fluorescence and Rayleigh diffusion. For low absorption, the intensity of the Rayleigh backscatter is proportional to the molecular density and independent of the temperature in the tuning band of the ArF laser, as long as the temperature is less than 500 K. The Rayleigh diffusion makes it possible to obtain a spatial image of the density. To obtain the absolute value of the density, calibration is necessary. This measurement method consequently has limits.